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Upon completion of this chapter, the student should be able to answer the following questions: 1.
Describe the organization of cardiac muscle and how it meets the demands of the organ.
2.
Describe the molecular mechanisms involved in excitation-contraction coupling in cardiac muscle and its suitability for this organ.
3.
Describe the molecular mechanisms that lead to an increase in the force of contraction of the heart.
4.
Discuss the length-tension relationship and the force-velocity curve for cardiac muscle, including the molecular basis for both curves.
If the student has already completed on skeletal muscle, the student will be able to compare cardiac and skeletal muscle for each of the learning objectives just listed.
he function of the heart is to pump blood through the circulatory system, and this is accomplished by the highly organized contraction of cardiac muscle cells. Specifically, the cardiac muscle cells are connected together to form an electrical syncytium, with tight electrical and mechanical connections between adjacent cardiac muscle cells. An action potential initiated in a specialized region of the heart (e.g., the sinoatrial node) is therefore able to pass quickly throughout the heart to facilitate synchronized contraction of the cardiac muscle cells, which is important for the pumping action of the heart. Likewise, refilling of the heart requires synchronized relaxation of the heart; abnormal relaxation often results in pathological conditions.
This chapter begins with a description of the organization of cardiac muscle cells within the heart, including discussion of the tight electrical and mechanical connections. The mechanisms that underlie contraction, relaxation, and regulation of the force of contraction of cardiac muscle cells are also addressed. Although cardiac muscle and skeletal muscle are both striated muscles, they are significantly different in terms of organization, electrical and mechanical coupling, excitation-contraction coupling, and mechanisms to regulate the force of contraction. These differences are also highlighted.
Basic Organization of Cardiac Muscle Cells
Cardiac muscle cells are much smaller than skeletal muscle cells. Typically, cardiac muscle cells measure 10 µm in diameter and approximately 100 µm in length. As shown in